DNA oligomers deposited on Cu(lll) surfaces were observed at liquid nitrogen temperature: using a scanning tunneling microscope. The observed oligomers were pAAAAAAATTTTTTT (14mer), pTTTGGTTAACCAAA (14mer), pGGGGGTTTTTTTTT (15mer), and pAAAAAAAAAATTTTTTTTTT (20mer). The structure of the isolated oligomers adsorbed on the Cu surface varies with the length of the molecular chain. The isolated 20mer is adsorbed to aggregate three-dimensionally. The isolated 14mer and 15mer are extended on the surface, and the almost entire molecular chain touches the surface. A highly resolved image of the 20mer shows bright spots aligned in a row along a single-stranded DNA with the same periodicity as that of the nucleotide units, demonstrating that each bright spot is a nucleotide.